1. Field of the Invention
The present invention is directed to balloon catheters and systems for monitoring the inflation of balloon catheters. The invention is also directed to materials and methods used to manufacture balloon catheters.
2. Discussion of the Background
Many kinds of catheters with balloons, including urethral Foley catheters, embolectomy catheters, gastrostomy tubes, endotracheal tubes and tracheotomy tubes, are widely utilized in clinical applications. The balloons are used as inflatable retention devices or for closing ducts and passageways in the human body. These catheters have been manufactured using various materials including natural rubber, silicone rubber, polyurethane, nylon, and plasticized polyvinyl chloride. The inflation status of the main balloon (the term main balloon is sometimes used to clearly distinguish the main balloon from the pilot balloon) in the body is usually detected by the pressure sensed between two fingers at the pilot balloon which is in air tight connection with the main balloon and is located outside the human body. The thickness of the wall of the pilot balloon usually has not allowed visual indication of the inflation status of the main balloon. A pressure gauge is available to indicate the inflation pressure of the balloon of an endotracheal tube, but is not widely used in clinical situations because the pressure gauge is cumbersome and obstructs operations on the patient.
The endotracheal tube, the main tube, or the shaft and the balloon are generally made of flexible polyvinyl chloride. When an endotracheal tube is used for mechanical ventilation during anesthesia with nitrous oxide, the balloon placed in the trachea is known to gradually increase in the degree of its inflation and in its pressure because of the diffusion of nitrous oxide into the balloon caused by the difference of partial pressure across the balloon membrane. The increase in the pressure of the balloon may injure the tracheal tissue pressed by the balloon because of blockage of the blood micro-circulation in the tissue. Thus, frequent regulation of the inflation pressure is needed by using a pilot balloon.
A product, called the Brandt system, which has a large pilot balloon with a thin wall membrane to diffuse out the nitrous oxide into the air, is marketed by Mallinckrodt, but the endotracheal tube with this system is not widely used because of the difficulties in using the large pilot balloon.
Another product called the Lanz system, which was developed to automatically control the pressure of the balloon with a large pilot balloon made of a material with an early yield point at the stress-strain curve, is marketed by Mallinckrodt but is not widely used either, because of the difficulties caused by its large size. Flexible polyvinyl chloride has excellent mechanical properties for disposable medical products, but has less balloon elasticity and has problems with plasticizers and the generation of dioxines during improper incineration.
It has previously been proposed in Patel, U.S. Pat. No. 4,335,723, to make a catheter having a balloon made of thermoplastic elastomer composition comprising block copolymer (Kraton G-61650 or G-1652) in the form of end blocks of polystyrene and elastomeric center block of saturated hydrocarbon polymer, mineral oil, polypropylene and antidegradants. It is disclosed in this patent that the balloon can be made by injection molding or by blow molding a tube of the desired dimension and wall thickness, which is cut to length to provide a collar or sleeve capable of being slipped over the shaft or tube of the catheter and capable of being bonded thereto along the margins of the collar or sleeve by an appropriate adhesive. However, this patent did not disclose the composition of thermoplastic elastomer suitable for the blow molding of thin walled balloons and did not provide ways to manufacture thin walled balloons which would inflate evenly under a low pressure that is safe to the tissue of the trachea.
Inflatable cuff-type catheters were recently developed utilizing alternative materials to flexible polyvinyl chloride or thermoplastic elastomer compositions similar to that disclosed by the above patent. A thin walled balloon was successfully blow molded and bonded to a shaft. The balloon was inflated at a low pressure safe to the tissue of trachea. However, the inflation status of the balloon, invisible from the outside of the human body, was impossible to detect by the conventional pilot balloon, because the composition of the balloon was very resilient and the inflation pressure was low as long as the wall of the balloon was not unreasonably thick.
Balloon or cuff pressure indicators are known from Bruner, U.S. Pat. No. 4,016,885, and Elam, U.S. Pat. No. 4,134,407. The former patent proposed the use of an open-ended spring wound at the mid-section of the pilot balloon to allow the spring to expand to show the gas pressure or the expansion of the balloon. The latter patent proposed an elastomer pilot balloon housed in a rigid cage having a plurality of windows to indicate the state of inflation and the pressure of the internal main balloon. These patents have disclosed a pilot balloon with an inner shaft but have not considered the similarity of materials of the balloon and the pilot balloon and have not disclosed a composition to enable the blow molding of balloons.
It is an object of this invention to provide a pilot balloon assembly for a safer balloon catheter, in which the pilot balloon assembly can visually indicate the inflation status of a main balloon made of very resilient material.
According to the present invention, the above and other objects are addressed by providing a small pilot balloon which visually indicates the status of the inflation of the main balloon in the human body. The small pilot balloon inflates similarly to the main balloon. The pilot balloon is conveniently manufactured of substantially the same material and has substantially the same structure as the main balloon based on the discovery that a thin walled balloon can be manufactured by blow molding if the material has certain properties. The size of the pilot balloon, which is easily measurable with an attached scale, is proportional to the size of the balloon in the body.
This invention is useful for a catheter with a balloon or a tube with a cuff, especially as an endotracheal tube where the balloon or the cuff is manufactured from a patient friendly resilient material which includes a blend of hydrogenated styrenic thermoplastic elastomer of styrene-isoprene/butadiene-styrene block copolymer with polyethylene/polypropylene as a preferred material.